The present invention relates to a rotary veneer lathe for cutting a wood block to peel therefrom veneer for use in manufacturing glued laminated wood such as plywood, laminated veneer lumber (LVL), etc. The invention relates also to a method of cutting a wood block by such veneer lathe.
A conventional rotary veneer lathe, part of which is shown in FIGS. 18 and 19, is disclosed in, e.g., KOKAI publication or unexamined Japanese patent application publication No. 2002-46109. The veneer lathe shown in the drawings has an elongated veneer knife 101 mounted in a movable carriage (not shown) for peeling veneer V from a rotating wood block 115. Reference numeral 108 designates a peripheral drive system which is also mounted in the knife carriage and includes a shaft 107 extending in parallel to the knife 101 and driven to rotate by a motor (not shown). A number of spiked peripheral drive wheels 105 (only one wheel being shown in the drawings), each having on the circumferential periphery thereof a number of spikes or tooth-like projections 103, is fixedly mounted on the shaft 107 at a predetermined spaced interval in the axial direction of the shaft 107. The shaft 107 is driven by the motor to rotate the peripheral drive wheels 105 in arrow direction as shown in FIGS. 18 and 19 for driving the wood block 115 from its periphery.
The veneer lathe further has a number of pressure members 109 which is mounted in the knife carriage, each disposed between any two adjacent peripheral drive wheels 105. The pressure member 109 has at the distal end thereof a replaceable insert 9a for pressing against the peripheral surface of wood block 115 immediately upstream of the cutting edge of the knife 101 as seen in the direction in which the wood block 115 is rotated as indicated by arrow. A guide member 111 is also mounted in the knife carriage between any two adjacent drive wheels 105 for guiding peeled veneer V along the periphery of the spiked drive wheels 105. Immediately downstream of the guide member 111 is disposed a separating member 113 having a contact surface 113a extending so as to intersect an imaginary circle which passes the tip ends of the respective projections 103 of the peripheral drive wheel 105 for separating or disengaging veneer V from the projections 103 of the drive wheels 105.
Referring to FIG. 18, the spiked peripheral drive wheels 105 of the conventional rotary veneer lathe are set in the knife carriage with respect to the knife 101 such that the tip ends of those projections 103 which pierce deepest into the wood block 3 are spaced at a distance of, e.g., about 1.5 mm from an imaginary line X-X (FIG. 18) which is drawn vertically upward from the cutting edge of the knife 5 and is assumed as an approximate line along which the knife 101 would cut into the wood block 115 when the latter is rotated in arrow direction.
In operation of the veneer lathe, with the wood block 115 supported at the opposite axial ends thereof by spindles (not shown) being driven to rotate in arrow direction by the spindles or the spiked peripheral drive wheels 105, the knife carriage is moved to feed the knife 101 into the wood block 115 at a controlled feedrate thereby to peel by the veneer knife 101 a veneer strip or sheet V with a predetermined thickness from the rotating wood block 115.
It is well known to those skilled in the art that a veneer lathe having the peripheral drive system 108 as shown in FIG. 18 is advantageous in that an excessive force will not be applied to wood block, so that a block having a weak core portion can be cut smoothly down to a small core diameter. To be more specific, the above veneer lathe is so designed and arranged that the power for driving the spindles thereby to rotate wood block 115 for peeling veneer therefrom is only of such a magnitude that veneer with a small thickness of about 1.5 mm is peelable, but it is insufficient for peeling veneer whose thickness is larger than 1.5 mm and, therefore, the power which is necessary for cutting wood block 115 for peeling veneer therefrom is supplied to the block primarily by the spiked drive wheels 105 which are disposed on the periphery of wood block 115 as shown in FIG. 18.
In cutting a wood block for producing veneer with a thickness of, e.g., about 3 mm, the peripheral drive wheels 105 in rotation is engaged at the tooth-like projections 103 thereof with the peripheral surface of the wood block 115, as shown in FIG. 18, thus power for cutting veneer from the block 115 being supplied to the block 115 from the peripheral drive wheels 105. Therefore, a wood block which is supported at its weak core portion by spindles can be cut successfully down to a small core diameter without being broken in the middle of peeling.
It is noted that, when the peripheral drive wheels 105 rotate in arrow direction, the wood block 115 is not rotated instantly with the rotation of the drive wheels 105 because of the cutting resistance exerted by the veneer knife 101 cut into the wood block 115. The force of the peripheral drive wheels 105 acting at the projections 103 thereof on the wood block 115 for rotation is increased while elastically deforming the wood of the block 115 at the projections 103 of the drive wheels 105, and the wood block 115 begins to be rotated for veneer peeling when the above force is increased to exceed the cutting resistance. Thus, the periphery of the wood block 115 is moved slower than the projections 103 of the peripheral drive wheels 105 for the above elastic deformation of the wood block 115. Consequently, the peripheral speed of the spiked drive wheels 105 at the projections 103 thereof is higher than the traveling speed of veneer V peeled from wood block 115 at the location adjacent to the guide members 111, so that the veneer V is subjected to tensile force by the projections 103 and, therefore, the veneer V just peeled from the wood block 115 is formed with a number of splits extending in the direction of wood grain of the veneer V, or in the direction that is perpendicular to the direction in which the veneer V is moved along the periphery of the spiked drive wheels 105. The veneer sheet V moving past the guide members 111 is then brought into contact with the surfaces 113a of the separating members 113 and bent downward as shown in FIG. 18, where further splits are formed in the veneer V.
During initial period of veneer peeling operation before the wood block becomes substantially cylindrical, veneer strips of various narrow widths are produced which are curled or coiled into spiral shape. Such curled veneer strips are troublesome to handle in the subsequent processes, but formation of splits along the wood grain of such veneer strips is effective to minimize curling of veneer strips.
Once the wood block 115 has been rounded up or become substantially cylindrical, a continuous veneer sheet V is peeled from the block 115. When a veneer sheet with a continuous width and free from pierced markings made by the projections 103 of the peripheral drive wheels 105 is needed for use as face veneer of plywood, the peripheral drive wheels 105 are moved for retraction by any suitable actuator as indicated by oblique arrow in FIG. 18 to a position shown in FIG. 19 where the projections 103 engage neither with the wood block 115 nor the veneer sheet V.
In veneer peeling with the peripheral drive wheels 105 positioned as shown in FIG. 18, splits formed in the veneer V by the tensile force from the projections 103 when the veneer V moves past the guide members 111 may be extended or lengthened along the wood grain depending on the species of wood veneer. Such veneer is weak to tensile force and hence tend to be broken easily along the extended splits, thus seriously affecting the overall veneer yield.
If veneer peeling is done with the peripheral drive wheels 105 retracted as shown in FIG. 19, splits will not be formed, but no driving force is transmitted from the peripheral drive wheels 105 to the wood block 115. Therefore, the veneer lathe then becomes incapable of peeling veneer sheet with a thickness of, e.g., about 3 mm.
When a knotty coniferous wood block is cut for veneer production, the resulting veneer has in it many knots. If a knot in the veneer may move between any two adjacent guide members 111, the knot is pressed from above by the projections 103 of a peripheral drive wheel 105 and broken to be removed from the veneer, with the result that a veneer sheet is produced which has a defective void portion and, therefore, is unusable as face veneer of plywood or similar panel product.
Therefore, it is an object of the present invention to provide a veneer lathe and a method of cutting wood block by such veneer lathe which can solve the aforementioned problems.